1. Field of the Invention
This invention relates to a process for preparing an aromatic urethane (hereinafter referred to simply as urethane). More particularly, it relates to a process for preparing a urethane by interacting an aromatic nitro compound (hereinafter referred to simply as nitro compound), an alcohol and carbon monoxide in the presence of a selenium-containing catalyst.
2. Description of the Prior Art
Urethanes have been heretofore produced mainly by the reaction of isocyanates with alcohols. In recent years, the production of isocyanates has become difficult partly due to the lack and rising costs of starting materials therefor and partly due to high toxicity of intermediates when producing the isocyanates. As a result, there have been developed and proposed many novel processes for the production of urethanes without the use of isocyanates. However, these newly developed processes involve several serious problems and have not yet been reduced to practice on an industrial scale.
For example, U. S. Pat. No. 3,338,956 describes a process wherein urethanes are prepared from alcohols, carbon monoxide and nitro compounds by the use of a rhodium chlorocarbonyl catalyst. However, this process is economically disadvantageous in preparing highly pure urethane since urethanes are obtained only at low yield even when the urethanation reaction is effected by the use of a large amount of the catalyst over a long period of reaction time.
There has been further proposed a process in which urethanes are prepared by interacting hydroxyl group-containing organic compounds, carbon monoxide and nitro compounds in the presence of a catalyst composed of a carbonyl group-containing compound of a metal of Group VIII of the Periodic Table and coexisting with a promoter of a salt of a metal selected from those capable of existing in two or more valence states (German Pat. No. 1,543,051). However, this process is useless from an industrial point of view since the yield of urethane product is low even when mononitro compounds are used as starting material, whereas use of dinitro compounds as starting material results in an even lower yield.
Moreover, a process is known using palladium and a Lewis acid as catalyst (U.S. Pat. No. 3,531,512). In this process, urethanes are obtainable in some cases at a fairly high yield of 80%-90% even when dinitro compounds are used as starting material. In order to attain such high yield, however, it is necessary to effect the urethanation reaction under such severe reaction conditions as an initial carbon monoxide pressure of 190-350 kg/cm.sup.2 and at a temperature of 190.degree.-200.degree. C. In addition, the process involves an industrially serious drawback that the Lewis acid, e.g., ferric chloride, used as promoter exerts a considerable corrosive action on metallic materials such as iron, stainless steel or the like. Accordingly, it is essential to use a reactor made of glass or tantalum so as to realize the process industrially. The use of a glass or tantalum reactor under the above-mentioned high temperature and pressure conditions, however, presents further technical and economical difficulties.
Further, French Pat. No. 2,197,862 describes a process using in combination selenium or a compound thereof and a base or water as catalyst. The reaction conditions of the process are milder than those of processes which have been previously known. However, the process undesirably involves side production of a substantial amount of amino compounds even if the reaction conditions are modified within defined ranges. Consequently, the yield of urethane becomes low. For example, so far as dinitrotoluene is concerned, the yield is at the best as low as 72-73%, higher yields being almost impossible to attain. As will be understood from the above, the process of the French Patent is not necessarily satisfactory for industrial production of urethanes. It will be noted that the mechanism of the urethanation reaction is not described in the French Patent specification. There is accordingly a strong demand for a process for preparing urethanes at higher yields.